1. Field of the Invention
The present invention relates to a process for preparing .beta.-form copper phthalocyanine pigment composition from crude copper phthalocyanine, and to the pigment composition and its use. More specifically, the present invention relates to a process for preparing .beta.-form copper phthalocyanine pigment by treating crude copper phthalocyanine with a printing ink resin or a printing in resin containing a solvent, to the pigment composition itself and the process for preparing printing ink from this composition.
2. Description of the Prior Art
Production of printing ink containing a pigment generally requires the following two processes:
The first process is one known as a pigmentation process. Copper phthalocyanine as synthesized is called crude copper phthalocyanine, and is not suitable as a printing ink pigment without further treatment since its .beta.-form crystal particles have large diameters in the range of 10 to 200 .mu.m. The process to reduce the particle size of this crude copper phthalocyanine to a size (from 0.02 to 0.1 .mu.m) suitable for printing ink is called pigmentation. Various methods for pigmentation are known in the art.
The method most generally employed is called the solvent salt milling method. This method involves adding to crude copper phthalocyanine a combination of milling agent such as sodium chloride and organic solvents which promote crystal transformation to .beta.-form and then carrying out the milling process. The .beta.-form copper phthalocyanine pigment obtained by this method has an aspect ratio (ratio of major diameter to minor diameter of primary particle) from 1 to 3, is suitable for printing ink because of its greenish hue, clearness and high color strength and thus is widely used. In this method, however, the quantity of milling agent required is several times that of the pigment, and considerable amount of time, labor and energy and expended to recover the milling agent and organic solvents.
Another conventional method involves dry-milling crude copper phthalocyanine before it is treated with an organic solvent or other chemicals. In this method, a part of the .beta.-form crystal undergoes transformation to the .alpha.-form as a result of mechanical stress during milling. To restore .beta.-form, therefore, the milled material must be subjected to heat treatment with an organic solvent. While this method has cost advantage over the solvent salt milling method, there are some offsetting problems such as the large aspect ratio of the particles as a consequence of their acicular growth during the heat treatment with an organic solvent, resulting in reddish hue and inadequate fluidity of the product. A method exists to suppress such particle growth by adding pigment derivatives and growth inhibitors. These additives, however, are often not desirable as components of printing ink.
The second process is known as an ink-making process. There are generally two methods for preparing printing ink from a pigment, one employs a dry pigment and the other a wet cake pigment usually containing 40 to 70% water. The ink-making method using a dry pigment is one in which a dry pigment is first mixed with a printing ink varnish, solvent, and other additives and then the pigment is dispersed by a device such as a beads mill, or a three-roll mill. Since the primary particles of a dry pigment has a strong agglomerate tendency considerable amount of energy is expended in an effort to disperse the pigment. The method using a wet-cake pigment is called the flushing method. It is a method in which a wet cake pigment is mixed with a printing ink varnish, solvent and other additives to change the phase of the pigment from the water phase to the varnish phase. Though this method requires no energy unlike the ink-making process of a dry pigment, a large-scale apparatus such as a kneader is required and discharged water produced by the flushing process imposes some difficulty.
As described in the preceding section, both the pigmentation process and the ink-making process require expenditure of considerable time and energy for the preparation of printing ink containing .beta.-form copper phthalocyanine. To provide printing ink at low cost, therefore, it would be desirable and efficient to produce ink directly from crude copper phthalocyanine without going through the manipulation of pigments. A method is known in which crude copper phthalocyanine is mixed with printing ink varnish and both the pigmentation and ink-making processes are accomplished simultaneously in a beads mill. However, since the pigmentation process in the printing ink varnish has low milling efficiency, it requires the use of a dispersion mill with ultra-fine beads and one is thus confronted with problems associated with energy efficiency and quality of the final product.
Japanese Patent Application Publication (JP-B) No. 55-6670 proposes a method in which crude copper phthalocyanine is first dry-milled and then made into ink without further steps. Though dry-milling is an effective method since it is inherently efficient, the milled material thus obtained is a mixture of .alpha.-form and .beta.-form crystals. The .alpha.-form crystal in the mixture must then be reconverted to the .beta.-form in the ink. Form transformation from the .alpha.-form to the .beta.-form progresses in the presence of heat and organic solvents and/or proceeds effectively in gravure ink which is rich in aromatic organic solvents. It is not as effective, however, in offset printing ink which contains only a small amount of solvent. Transformation to the .beta.-form is very difficult in solvents containing no aromatic compound (AF solvent) that are becoming increasingly popular.
In this regard, it is known that reduction of agglomeration of milled copper phthalocyanine is an effective means of accomplishing ink-making of this milled material efficiently. Several methods have been proposed. For example, British Patent 1224627 proposes a method in which dry-milling of crude copper phthalocyanine one part is accomplished with the addition of one part to 8 parts of resin. Japanese Patent Application Laid-Open (JP-A) No. 2-294365 proposes a method in which 0.5 to 10% of resin such as a rosin modified phenolic resin is added for the dry-milling of crude copper phthalocyanine. These methods are effective in preventing agglomeration of copper phthalocyanine particles in the presence of the added resin. The rosin modified phenolic resin, however, is known for its tendency to degrade by oxidation even when it is kept at room temperature. For the methods proposed in these patents, it is impossible even under cooling to prevent oxidation of the resin additives during milling, since milling causes increases in surface area and collisional impact generates thermal spikes. Since oxidation changes the solubility, color tone and the other properties of the resins, choosing conventionally used resins for this purpose cannot yield products with characteristics comparable to the conventional ink.
Furthermore, the primary particles of the ink obtained by these methods are characterized by their acicular shape unlike those obtained by the solvent salt milling method. Thus, problems of reddish hue and of fluidity remain unresolved. A known method exists to suppress this particle growth. It involves addition of pigment derivatives and growth inhibitors. These additives, however, are often not desirable as the components of final printing ink.